Tunable near- to mid-infrared (IR) ultrafast optical pulses have a variety of applications in scientific, metrology and engineering contexts, including time-resolved spectroscopy, frequency-resolved spectroscopy, nonlinear spectroscopy, laser waveguide inscription, nonlinear microscopy and laser machining.
Two sources of such pulses are known in the prior art: (a) low-energy synchronously-pumped optical parametric oscillators (OPOs), operating at energies of typically <5 nJ and repetition rates of typically 80 MHz; and (b) optical parametric amplifiers pumped by amplified Ti:sapphire lasers, delivering repetition-rates in the 1 kHz-200 kHz range and energies in the 0.1-100 μJ range. For many applications there is an energy range (for example, 10-100 nJ) and repetition-rate (for example several MHz to several 10s of MHz) range that is desirable but which is not readily accessible to either of the conventional approaches.
There have been attempts to extend the operational range of OPOs. For example, C. Min et al, “Near-infrared cavity-dumped femtosecond optical parametric oscillator,” Opt. Lett. 30, 1855-1857 (2005) describes the production of 90 nJ signal pulses in the near-IR region, at 1200 nm, using a cavity-dumped high-repetition-rate (82 MHz) OPO. However, the repetition rate of the signal pulses was not within the above-mentioned region, being 1 MHz, and the average pulse power was relatively low, at 90 mW. The idler pulses produced by the OPO were of low energy, at around 1 nJ, and were produced at the repetition rate of 82 MHz.
T. Südmeyer et al, “High-power femtosecond fiber-feedback optical parametric oscillator based on periodically poled stoichiometric LiTaO3,” Opt. Lett. 29, 1111-1113 (2004) describes a fiber-feedback OPO that was synchronously pumped by a 58 W thin-disc laser and that was described as achieving 339 nJ output pulses at 1450 nm, at a repetition frequency of 56 MHz. However, the system described in T. Südmeyer et al requires a high average power complex pump laser (a 56 W thin-disc layer), which requires a chilled-water cooling system and includes a sophisticated, free-space array of multiple-pass mirrors, uses fibre feedback cavity with low cavity finesse, is not continuously tunable and whose output does not provide a diffraction-limited output beam (M2 is 1.6). The output wavelength of the system described in T. Südmeyer et al is principally determined by the phase-matching range of the non-linear crystal that is used in the system.
Methods and apparatus are required that can directly provide tunable, high-energy near- to mid-infrared pulses at multi-MHz repetition rates, without the complexity and expense of ultrafast amplifier systems, are desirable.
It is an aim of the present invention to provide an improved, or at least alternative, optical apparatus and method.